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By Dr. Daniel Ziskin

Everybody knows what weather means. What’s the temperature? Is it raining? Snowing? Just poke your head outside and you’ve got weather. Weather is the instantaneous atmospheric conditions we experience. Climate, however, is something different. Climate is the average of the weather over time and space. But taking the average of a constantly changing and location-specific phenomenon is complex. Are we talking about a monthly average? Seasonal? Yearly? Of a county? A state? The northern hemisphere, or globally? Climate depends on the temporal and spatial domain you’ve selected so there isn’t just one climate.

To further complicate the issue, suppose we are talking about the global mean of annual surface temperature. If it is stated that this value is increasing by one degree, it doesn’t mean that it is increasing everywhere by that amount. Some places will be getting much warmer than other places. Some places, due to shifts in wind and precipitation patterns, might even cool a little bit. In terms of the observed climate change which will be described below, it is also quite likely that we will see colder winters even as the average temperature increases.

Basic concepts

Climate change refers to a drift in the average temperature over time. What would cause the average temperature to change with time? A stable climate exists when heating and cooling rates are in balance. They are almost never in balance at any one time or place. It’s usually getting hotter or colder at any one place. But globally, over a long enough time span, if heating and cooling are balanced then the average temperature remains constant. Over the last 1,000 years, we have seen a somewhat stable global average temperature, but there has been a rapid increase in temperature over the last century (see Figure 1). This observed temperature increase in climate indicates that the heating of the planet is no longer in balance with the cooling.

Climate change is more than just temperature. It also includes issues such as precipitation patterns (such as droughts or monsoons), wind, vegetation, wildlife habitat and risk of wildfires. One particular ancillary consequence of climate change is in the frequency and severity of storms. Storms are a result of instability of atmospheric conditions. For example, if a mass of cold dense air aloft is supported by warm buoyant air near the surface it can become gravitationally unstable. The heavier cold air falls and the warm surface air rises. This is called convection and usually causes massive downpours and thunderstorms. In a warming climate, where the surface air is heated beyond current rates, we can expect more of these types of storms and they are predicted to be more intense.

Heating

The fundamental heat source of the Earth is the sun. The sun’s rays arrive at the top of the Earth’s atmosphere. About a third of the sunlight hits the tops of clouds and is immediately reflected back out to space. The rest passes through the atmosphere and hits the surface. When the sunlight hits the surface, some of the light is reflected back out and the rest is absorbed. The absorbed portion is the energy that heats the Earth.

The amount of sunlight that is absorbed compared to what is reflected depends on the surface reflectivity. For example, cement is reflective so little energy is absorbed, whereas asphalt is dark and absorbs more sunlight. Test this yourself on a hot summer day. You might be able to walk barefoot on the sidewalk when the blacktop of the street is painfully hot. This happens on a global scale, too. Dark surfaces like the ocean absorb a lot of heat. Bright surfaces like snow reflect a lot.

Although there are some variations in the sun’s energy output, primarily due to the 11-year solar cycle, it is generally a constant source of heat. Solar variations cannot explain the observed climate change as is sometimes claimed. Changes in cloudiness and changes in land use have a larger effect on the heating rate of the Earth than the solar cycle. Likewise, there are temporary disruptions due to events like volcanoes or smoke from enormous fires that tend to reduce the heating rate by creating large artificial clouds that reflect light. The heating of the Earth at the top of the atmosphere is, for the scope of this article, constant. However, the amount of light that is reflected relative to the amount absorbed is changing. This energy balance is illustrated in Figure 2.

Cooling

As the sun heats the planet’s surface, the Earth cools itself by sending off energy into space. The cooling has to stay in balance with the heating or the temperature will change. The surface gives off heat and the hotter the surface temperature, the more heat is given off. Of the heat that passes through the atmosphere, some of it escapes to space and some is absorbed by the atmosphere. When the energy is absorbed by the atmosphere it is re-emitted both up and down. The downward component contributes to the heating of the surface while the upward component is cooling (See Figure 3).

How much heat is absorbed by the atmosphere? That depends on the amount of greenhouse gases. Greenhouse gases are constituents of the atmosphere that absorb and trap heat. The most common is water vapor (about 2 percent of the atmosphere). Water vapor is the cause of most of the greenhouse effect. However, there are some wavelengths of infrared light that can pass through the water vapor and escape to space, cooling the surface. These “windows” in the infrared spectrum are transparent, just like the atmosphere is transparent to visible sunlight.

So if water vapor is the major greenhouse gas, what’s the big deal about carbon dioxide or methane or other greenhouse gases? Carbon dioxide exists at a much lower concentration of the atmosphere — about 0.4 percent — than water vapor, but it is a powerful greenhouse gas because it absorbs light in the window regions of the spectrum that water vapor doesn’t. So a little bit of carbon dioxide absorbs and re-emits a lot of heat.

The concentration of carbon dioxide has increased from about 280 parts per million at the beginning of the Industrial Revolution in the mid-18th century to a 2011 value of 392 parts per million. Climate scientists are convinced that the increased concentration of carbon dioxide in the atmosphere is the major cause of the recent increase in temperature that is shown in Figure 1.

Carbon dioxide is a naturally occurring gas in the atmosphere. As stated above, it existed at a concentration of about 280 parts per million prior to humanity’s decision to utilize the energy embedded in fossil fuels — coal, methane and petroleum. There are massive exchanges of this gas between the ocean and atmosphere and between the plants of the world. However, when we burn fossil fuels we add a small but significant unbalanced contribution to the atmosphere.

Let’s use a bank account as an analogy. If every month you deposit about $5,000, and your monthly expenses are $4,990, every month you generate a net positive amount of $10. Even though that’s small relative to the flow of money, over time the average balance will increase. That’s what’s happening to the concentration of carbon dioxide in the atmosphere. The concentration seems to be increasing by about 2 parts per million, or less than 1 percent, per year.

Much ado is made these days about the number 350 parts per million of carbon dioxide (i.e. 350.org). This number is significant because it is believed to be the largest concentration we can sustain and still avoid catastrophic consequences of climate change (such as melting of the polar ice caps). The fact that we have significantly exceeded this number — our atmosphere stood at 393 parts per million as of 2011 — suggests that modestly reducing our emission rates will not avert severe climate change.

Predictions

It might seem as if climate science is all just basic physics, and it would be if the Earth was just a floating rock in space like the moon. But instead, the Earth is a complex and dynamic place. Positive feedback loops are ways in which small disturbances are amplified into large signals. We are all unfortunately familiar with the screech of microphone feedback. That occurs when a sound is amplified and the microphone picks up that sound and amplifies it again, and then it picks up the amplified sound and amplifies it again, and so on ad nauseam. The Earth’s climate also has similar positive feedback loops. Here are two examples:

The sea ice that covers the Arctic Ocean is melting in the summer time. As more dark water is exposed, as compared to reflective sea ice, then more sunlight is absorbed. This leads to more heating and consequently more melting of the sea ice.

Massive reserves of methane — a potent greenhouse gas — are frozen in the Siberian tundra. As the climate warms, some of this frozen ground thaws, releasing the methane, which increases the greenhouse effect, further warming the planet, which in turn thaws more tundra.

The magnitude of responses such as these to the initial climate change we’ve observed so far is difficult to quantify, and that is why climate scientists are uncertain about the speed and ferocity of future climate change.

There also are other factors about the Earth’s climate system that confound simple predictions:

How much heat is being stored in the deep ocean rather than increasing the surface temperature?

Will the massive ice sheets on Greenland and Antarctica continue to melt slowly or will they slip quickly into the ocean?

Will global cloudiness change in response to a changing climate and, if so, how?

Will the patterns of precipitation change and, if so, how?

Will the changes in polar-ice melting change ocean-circulation patterns?

How will society respond to climate change? Will we significantly reduce our present and future emissions?

Questions such as these are the fuel for vigorous scientific research and make it challenging to provide a simple prediction about future climate change.

Despite the high degree of uncertainty about exactly what the future climate state may be, some trends are already discernable. In addition to warmer average surface temperatures, other worrisome observations include:

A rapidly shrinking summertime arctic ice cap (see Figure 4).

Sea-level rise (see Figure 5).

An increase in the severity of Atlantic hurricanes (see Figure 6).

Conclusion

There is little scientific debate about the basic physics of climate change. How the Earth’s climate system will respond to warming makes predictions of the future climate conditions uncertain. Heating is nearly constant, but cooling is diminished due to the increased concentration of greenhouse gases. This imbalance between heating and cooling is the cause of the observed increase in temperature relative to historic (pre-Industrial Revolution) values.

Dr. Daniel Ziskin is a former member of the board of directors of the Green Zionist Alliance. He earned his doctorate in physics at Johns Hopkins University, where he wrote his dissertation on climate change. Previously he worked at the National Oceanic and Atmospheric Administration’s National Geophysical Data Center, the National Center for Atmospheric Research, and the NASA Goddard Space Flight Center’s Distributed Active Archive Center.